In the prior art, numerous glass-ceramic materials and articles have been described, for use such as cooktops, cooking utensils, stove windows and the like.
In U.S. Pat. No. 5,446,008, glass-ceramics are described having improved transmission. With reference to this technical problem of transmission, it is taught that the water content of the glass-ceramic must be controlled. Said water content must in fact be less than 0.03 mol/l. Moreover, in this reference, the technical problems of mechanical strength and thermal expansion were not tackled.
In U.S. Pat. No. 4,461,839, other glass-ceramics are described. Their composition, which is almost free from MgO, is not optimized with reference to said technical problems of mechanical strength and of thermal expansion.
In European patent application publication No. EP-A-1 170 264, the problem of surface cracking of the glass-ceramics is discussed and, with reference to said problem, novel glass-ceramics are proposed which are original through their composition and their state of crystallization.
U.S. Pat. No. 5,070,045 describes glass-ceramic materials having β-spodumene or β-quartz solid solution as the predominant crystalline phase. The glass-ceramic materials comprising β-spodumene solid solution as the predominant crystalline phase have an opaque, white or colored, appearance. They are opaque in the visible, in order to prevent the users of plates from being dazzled by heating elements which are placed under such plates, but they keep a high transmission in the infrared, so as to improve the heating of the cooking utensils which are placed on said plates. Said utensils are heated not only by conduction but also by radiation. The glass-ceramic mateiral is obtained by heat treatment of the glass precursor, which comprises a step of crystallization at a temperature on the order of 1,070° C. to 1,090° C. The higher the temperature of this crystallization step is, the more the opacity of the material.
These glass-ceramics and the method of obtaining them are described more specifically in the patent application WO-A-99 06334.
Though the materials have been used in widely commercialized glass-ceramic products, there is still room for improvement:
a) said plates of β-spodumene are less efficient than their β-quartz counterparts, which are also described in U.S. Pat. No. 5,070,045, in terms of mechanical strength and thermal expansion; and
b) where more opaque products are desired (e.g., for use in a context of heating by induction), it is suitable to ceramize the glass precursor at a higher temperature (≧+20° C.). As a result, cracks can be generated in the structure of the glass-ceramic. In the past, it was observed that a lower mechanical strength and the presence of cracks, when the ceramization temperature is increased, were linked to the presence of water content in the glass precursor. It is known that the water can be incorporated in said glass precursor during its melting. The water content of said glass does in fact depend upon the state of the starting materials used and upon the means of melting used. Thus, if the precursor glasses of the glass-ceramics are melted from starting materials which contain little water in an electric oven, the resulting glass-ceramics tend to have a high mechanical strength and do not have a tendency to crack, even if the ceramization is carried out at high temperature. Glasses having low water content can similarly be used by adding halogens into their basic composition. This is described in U.S. Pat. No. 5,446,008. Industrially however, it is highly desired to not have to be concerned about controlling the water content of the glass. It is notably desired to be able to implement methods of melting employing burners of air-natural gas type or oxygen-natural gas type. The person skilled in the art cannot ignore that the use of such means of melting lead to significant contents of water in the glasses. It is the water contents which limit the ceramization temperatures.
Thus, in accordance with the invention, the improvement sought after to said β-spodumene glass-ceramic materials has been along two lines. It was desired both to increase the mechanical strength and to avoid the appearance of cracks (in the case of glasses which contain much water and/or for ceramizations at high temperature) and to decrease the thermal expansion.
Furthermore, the glass precursor of the glass-ceramics of the invention are also suitable for obtaining efficient glass-ceramics of main crystalline phase of β-quartz. It therefore enables the manufacture of various transparent or opaque products from the same glass precursor composition. This flexibility of the method in question is highly desirable in the art.
Said person skilled in the art has therefore already understood the interest of the present invention, the main subject of which consists of novel glass-ceramic compositions which are suitable for efficient β-quartz and β-spodumene glass-ceramics, more particularly β-spodumene glass-ceramics which are more efficient than those of the prior art (U.S. Pat. No. 5,070,045 and WO-A-99 06334).